(a) Field of the Invention
The invention relates to an improved hydraulic quick lifting unit used in a jack, especially to improvements made on the valves of the hydraulic system, a fastener assembly and an oil return tube of the lifting shaft. The apparatus according to the invention can provide a high accurate clearance between the lifting shaft and the mandrel and two returning speeds of the lifting shaft depending on its loading.
(b) Description of the Prior Art
Conventional one step hydraulic jacks are operated by starting a pump to drive the lifting shaft from an original position (or the lowest position) to an arbitrary position within the maximum travelling range. U.S. Pat. Nos. 3,581,499, 2,702,988, 4,339,942 and U.S. Pat. No. 5,755,099 have disclosed such apparatuses. The lifting shaft of the above apparatus retracts from the highest position only with a low speed either under a heavy load or without a load on the top of the lifting shaft. However, it is desirable to let the lifting shaft retract at a higher speed as the loading on the shaft is removed. Besides, the apparatus disclosed in U.S. Pat. No. 5,755,099 has such a structure in which there exists a clearance between the mandrel and the inner oil reservoir of the lifting shaft due to the requirement of machining components. The clearance is about 0.4 millimeter. However, there is not any holding means to associate the mandrel with the inner oil reservoir. It is inevitable to shift position after a certain use duration. This will lead to damages of O-rings and back rings and oil leakage. It affects the reliability of product performance very much.